1. Field of the Invention
The present invention relates, in general, to temperature sensors, and in particular to a new and useful etalon/fiber optic based temperature sensor.
2. Description of the Related Art
A Fabry-Perot etalon consists of two parallel planar reflecting surfaces separated by a distance .tau.. Due to interference among the multiply reflected beams from the two reflecting surfaces, the reflectance of an etalon is a strong function of wavelength and the optical path length, n.tau.; where n is the index of refraction of the medium between the two surfaces.
Mathematically the reflectance can be written as: ##EQU1##
Where
r.sub.1 is the reflectance of the first surface PA1 r.sub.2 is the reflectance of the second surface PA1 r=(r.sub.1.cndot.r.sub.2).sup.1/2 and .delta.=the phase difference between two successive beams and is given by ##EQU2##
when the illuminator is a collimated beam of wavelength=.lambda..sub.o, incident perpendicular to the reflective surfaces.
FIGS. 1 and 2 show how the reflectance changes as a function of and .tau. and .lambda..sub.o, respectively.
The three references L. Schultheis, H. Amstutz, and M. Kaufmann, "Fiber Optic Temperature Sensing With Ultrathin Silicon Etalons," Optics Letters 13, No. 9, Sep. 1988, p. 782; J. W. Berthold, S. E. Reed, and R. G. Sarkis "Simple, Repeatable, Fiber Optic Intensity Sensor for Temperature Measurement," SPIE OE Fibers '89 Proceedings, Vol. 1169; and J. C. Hartl, E. W. Saaski, and G. L. Mitchell, "Fiber Optic Temperature Sensor Using Spectral Modulation," SPIE Vol. 838, Fiber Optic & Laser Sensors V (1987), p. 257; describe temperature sensors that use a thin silicon etalon deposited on the end of an optical fiber. Components of those sensors are shown schematically in FIG. 3.
These sensors consist of:
a narrow band light source 10; an optical fiber 12 that carries the light to the thin film (.tau.=500 to 1000 nm) silicon etalon 14; a 2.times.2 fiber optic coupler 16 which serves to divert half of the outgoing light to a reference detector 18 and half of the light reflected from the etalon to a second detector 20; and electronics 22 that ratios the reflected signal I to the reference signal I.sub.o to determine the reflectance of the etalon R=I/I.sub.0.
The index of refraction of silicon decreases with temperature causing a change in phase, .delta.. The resultant effect on the reflectance is shown in FIG. 4 for an etalon with room temperature thickness of 785 nm. The temperature also changes the thickness (thermal expansion) of the etalon but the magnitude is negligible in comparison to the index change.
It can be seen in FIG. 4 that the measured reflectance is a single valued function only over the restricted range from -100.degree. C. to about 4000.degree. C. Actually, the limits of this range can be shifted by changing either the wavelength or the thickness, but the upper and lower limits move together yielding a fixed range of about 5000C.
The reference J. C. Hartl, E. W. Saaski, and G. L. Mitchell, "Fiber Optic Temperature Sensor Using Spectral Modulation," SPIE Vol. 838, Fiber Optic & Laser Sensors V (1987), p. 257 describes a similar sensor where instead of using a reference detector to compensate for source variations, the signal beam reflected from the etalon is separated into two wavelength bands (both within the narrow 100 nm band of the LED source) by a dichroic beam splitter. The two bands are detected and the ratio of the two provides a signal that is dependent on temperature, but insensitive to changes in source intensity, fiber transmission or connector loss. This approach also yields a range of about 5000.degree. C.